The present patent application relates to a wind turbine having a rotor with a first rotor blade and a pitch control system. Further, present patent application relates to a controller for controlling a pitch angle of at least one rotor blade of a wind turbine. Finally, the present patent application relates to a method for controlling a pitch velocity of a rotor blade of a wind turbine.
In case of an emergency event, e.g. the loss of the grid, a wind turbine has to be stopped as fast as possible. Therefore, rotor blades of a wind turbine are rotated from a working position into a feathered position. For this purpose emergency pitch systems are used. The velocity of the rotation is often not controlled since in many emergency events a pitch controller is not powered and therefore not in operation. The desired velocity for rotating the rotor blade from an operating position to a feathered position is primarily taken to reduce the power output as quick as possible, not to reduce the turbine loads resulting from this action. For example if the rotor blades are rotated too fast from the operating position into the safety or feathered position, the rotor is still rotating with relatively high speed when the rotor blades are brought into the feathered position. In this case high forces may be created that suck the rotor into the direction of the wind so that the wind turbine might be damaged. Thus, in previous wind turbines the pitch velocity is fixed or limited such that such large forces are avoided. These fixed pitch velocities are normally very low, e.g. 8° per second. As the at the time of the design and construction of the wind turbine the circumstances of emergency events are obviously not known, the fixed pitch velocities are independent from the wind conditions and meet worst case scenarios. For limiting the pitch velocity, emergency pitch systems often contain a safety brake system.
Other known emergency pitch systems are provided with energy storage devices, e.g. accumulators, batteries or capacitors. The energy storage devices power the pitch controller during a grid outage and enable a rotation of the rotor blade from the operating position into the feathered position. If accumulators or capacitors are used, they are charged during normal operation of the wind turbine.
Further, specific embodiments of known emergency systems have a direct electrical connection between the energy storage devices and a pitch motor that drives the rotor blade. In the latter case the pitch velocity is uncontrolled since the velocity of the pitch system depends on the counter torque it has to overcome and energy storage charge/performance status of the energy storage device, e.g. the battery or the accumulator.